Percentage Difference
The percentage difference is:
The difference between two values divided by the average of the two values. Shown as a percentage.
Difference means to subtract one value from another:
Example: Alex sold 15 tickets, and Sam sold 25
The difference between 25 and 15 is: 25-15 = 10
Average is the value half-way between:
average = | first value + second value |
2 |
Example continued
The average of 25 and 15 is: (25+15)/2 = 40/2 = 20
And then the difference as a Percentage of the average:
Example continued
Difference is 25-15 = 10
Average is (25+15)/2 = 20
10 as a percentage of 20 is:
10 |
× 100% = 50% |
20 |
The percentage difference between 25 and 15 is 50%
Here is the answer, in one line:
| | 25 - 15 |
| | × 100% = 50% |
(25 + 15)/2 |
Now let's find out when, why and how to use it ...
When Should I Use It?
Percentage Difference is used when both values mean the same kind of thing (for example the heights of two people).
Neither value is more important than the other.
- But when there is an old value and a new value, you should use Percentage Change
- Or when there is an approximate value and an exact value, you should use Percentage Error
Why Use The Average?
Because there is no obvious way of choosing which value is the "reference" value.
Example continued
- If we used "15" we would get 10/15 = 66.6...%
- If we used "25" we would get 10/25 = 40%
But which one should we use? And if someone else did the calculations which one would they use?
So it is best to choose a value half-way between so there is no confusion.
What if the Difference is Negative?
We can't say which value is more important, so we can't say if the difference is "up" (positive) or "down" (negative) ... so we simply ignore any minus sign.
Example: Alex works 6 hours, and Sam works 9 hours
Difference = 6-9 = -3
But in this case we ignore the minus sign, so we say the difference is simply 3
(We could have done the calculation as 9-6=3 anyway,
as neither Sam nor Alex is more important!)
The Average is (6+9)/2 = 7.5
Percentage Difference = (3/7.5) x 100% = 40%
How to Calculate
Step 1: Calculate the difference (subtract one value form the other) ignore any negative sign |
Step 2: Calculate the average (add the values, then divide by 2) |
Step 3: Divide the difference by the average |
Step 3: Convert that to a percentage (by multiplying by 100 and adding a "%" sign) |
Examples
Example: Juice costs $4 in one shop and $6 in another shop, what is the percentage difference?
- Step 1: The difference is 4-6 = -2, but ignore the minus sign: difference=2
- Step 2: The average is (4+6)/2 = 10/2 = 5
- Step 2: Divide: 2/5 = 0.4
- Step 3: Convert 0.4 to percentage: 0.4×100 = 40%.
The percentage difference is 40%
Another Example: There were 160 smarties in one box, and 116 in another box, what is the percentage difference?
160 to 116 is a difference of 44.
Average is (160+116)/2 = 276/2 = 138
44/138 = 0.319 (rounded to 3 places) = 31.9%
The Formula
You can also put the values into this formula:
| | First Value - Second Value |
| | × 100% | |
(First Value + Second Value)/2 |
(The "|" symbols mean absolute value, so any negatives become positive)
Example: "Best Shoes" gets 200 customers, and "Cheap Shoes" gets 240 customers:
| | 240 - 200 |
| | × 100% = |40/220| × 100% = 18.18...% | |
(200+240)/2 |
An interesting thing about this formula is that it doesn't matter which is the 1st or 2nd Value:
Put the values the other way around:
| | 200 - 240 |
| | × 100% = |-40/220| × 100% = 18.18...% | |
(240+200)/2 |
The answer is the same (because we take the absolute value).